Fuel cells and more specifically Molten Carbonate Fuel Cells are well known in the art (J. R. Selman, T. D. Claar, "Proceedings of the Symposium on Molten CArbonate Fuel Cell Technology" Proceedings Volume 84-13, The Electrochemical Society, Inc.). In a fuel cell, chemical energy is converted directly into electrical energy. A fuel cell comprises electrodes: a cathode and an anode. The electrodes act as catalytic reaction sites where the fuel and oxidants are electrochemically transformed into electricity, water or carbon dioxide and heat. The electricity being produced as D.C. is conveniently transformed into A.C. before utilization, e.g. to fulfil the need (or some of the need) of a process plant.
The use of hydrogen-containing purge gas from a synthesis process is further disclosed in French patent specification No. 2,374,752 (Pinto). By the process there disclosed purge gas from, e.g., ammonia synthesis and containing only hydrogen, nitrogen, noble gases and methane is oxidized in a fuel cell to generate electricity. Residual gas from the fuel cell is recycled as fuel or process feed.
At the anode, fuel is oxidized electrochemically to give up electrons which are conducted through an external circuit to the cathode where the elecrons combine with the oxidant. The loop is closed by ions which are conducted through an electrolyte from one electrode to the other.
Molten Carbonate Fuel Cells are known in two principally different forms, i.e. simple Molten Carbonate Fuel Cells (MCFC) and Internal Reforming Molten Carbonate Fuel Cells (IRMCFC). The main characteristics of these two forms of Molten Carbonate Fuel Cell are given in Table 1:
TABLE 1 __________________________________________________________________________ MCFC IRMCFC __________________________________________________________________________ Electrolyte K.sub.2 CO.sub.3 --Li.sub.2 CO.sub.3 K.sub.2 CO.sub.3 --Li.sub.2 CO.sub.3 Electrolyte support LiAlO.sub.2 LiAlO.sub.2 Electrodes (catalysts) Ni, NiO Ni, NiO Anode fuel H.sub.2, CO Natural gas, H.sub.2, CO Cathode oxidant Air + CO.sub.2 Air + CO.sub.2 Temp., .degree.C. 600-700 600-700 Pressure &lt;120 psia [&lt;8.437 kg/cm.sup.2 abs.] &lt;120 psia [&lt;8.437 kg/cm.sup.2 abs.] Cell voltage, V &lt;0.85 &lt;0.85 Impurity tolerance No H.sub.2 S No H.sub.2 S Anodic reaction H.sub.2 + CO.sub.3.sup.-2 .fwdarw. CH.sub.4 + 2H.sub.2 O .fwdarw. CO.sub.2 + 4H.sub.2 (example) H.sub.2 O(g) + CO.sub.2 + 2e.sup.- 4H.sub.2 + 4CO.sub.3.sup.-- .fwdarw. 4CO.sub.2 + 4H.sub.2 O + 8e.sup.- Total CH.sub.4 + 4CO.sub.3.sup.-- .fwdarw. 5CO.sub.2 + 2H.sub.2 O + 8e.sup.- Cathodic reaction (example) CO.sub.2 + 1/2O.sub.2 + 2e.sup.- .fwdarw. CO.sub.3.sup.-2 4CO.sub.2 + 2O.sub.2 + 8e.sup.- .fwdarw. 4CO.sub.3.sup.-- Overall reaction H.sub.2 + 1/2O.sub.2 .fwdarw. H.sub.2 O(g) CH.sub.4 + 2O.sub.2 .fwdarw. CO.sub.2 + 2H.sub.2 O Fuel used in example H.sub.2 CH.sub.4 __________________________________________________________________________
An MCFC uses hydrogen and/or carbon monoxide as fuel and needs an oxidant comprising oxygen (air) and carbon dioxide as shown in Table 1.
The production of electricity will cause a migration of carbonate ions from the cathode to the anode, and carbon dioxide is produced at the anode and consumed at the cathode. Therefore, a continuous transfer of carbon dioxide from the anode back to the cathode--"carbon dioxide sweeping"--is required. The carbon dioxide sweeping is usually accomplished by post combustion of the anode exhaust gas. This post combustion causes an appreciable reduction of the obtainable conversion of fuel into electricity.
As shown in Table 1, an IRMCFC can use methane (natural gas), hydrogen and/or carbon monoxide as fuel and needs an oxidant containing oxygen (air) and carbon dioxide.
At the anode, carbon dioxide is produced. An IRMCFC isolated from a chemical process plant will need "carbon dioxide sweeping" in much the same way as an MCFC.
The term "fuel cell" as used hereinafter means stacks of fuel cells. Normally, fuel cells are used in series, stacks, in order to provide for a sufficient voltage.
The combination or integration of chemical and fuel cell process units is known in principle.
J. H. Altseimer et al. (Fuel Cell Seminar, Oct. 26-29, 1986 Sheraton El Conquistador Tucson, Ariz.) mention the application of fuel cells to the petroleum refining industry as a supplemental energy source to existing power and steam generating systems. However, no further description of the integration of fuel cells in the refinery processes is given.
It has further been proposed to integrate a heat exchange reformer such as described for instance in European patent specification No. 195,688 and a fuel cell (a phosphoric acid fuel cell). According to this concept, fuel in the form of natural gas is reformed in a reaction with steam into hydrogen which is utilized in the fuel cell for production of electrical power. The anode exhaust gas is utilized as fuel in the reformer, and steam raised from the fuel cell waste heat is used for the reforming process.
EP Patent Application No. A2-0-170277 discloses a power plant comprising a molten carbonate fuel cell, a reformer for reforming fuel into a reaction gas for the anode of the cell and a combustor to burn the exhaust gas from the anode and to supply the burnt exhaust gas to the cathode.
U.S. Pat. No. 4,522,894 (Hwang et al.) describes a fuel cell power plant using an autothermal reforming process on-site using generation of the hydrogen-rich fuel to be supplied to the anode side of the fuel cell. The cathode vent gas is fed to the autothermal reactor and the anode ent gas is fed to the catalytic burner to preheat the inlet stream to the reformer.
U.S. Pat. No. 3,488,226 (Baker et al.) shows a process for the generation of hydrogen from hydrocarbons by low pressure steam reforming and use thereof in Molten Carbonate Fuel Cells. The reforming reaction is carried out in a heat exchange relationsship with the fuel cell whereby the fuel cell heat sustains the endothermal reforming reaction. The reforming reaction step takes place on a catalyst located in the anode chamber of the fuel cell. The spent fuel from the anode is burnt off for providing further heat to the endothermal reforming reaction.
GB Patent Specification No. 1,309,517 (Fischer et al.) describes a fuel cell for transforming combustion enthalphy of hydrocarbons into electrical energy. Some of the exhaust gases generated flow together with combustion air not consumed in the modules through a porous catalyst where it is burnt in order to cover the heat requirements for the endothermal reforming reaction on the anodes of the fuel cell modules.
U.S. Pat. No. 4,524,113 (Lesieur) discloses the operating of a molten carbonate fuel cell by contacting the catalyst-containing anode of the fuel cell with methanol in the presence of water. The methanol is hereby caused to steam reforming inside the fuel cell hereby producing carbon monoxide, carbon dioxide and hydrogen, which is used as fuel for the anode.
Japanese Patent Specification No. 60-59672 (cf. Patent Abstract of Japan, abstract of JP 60-59672, vol. 9, No. 193, publ. 85-04-06) discloses a method having for its purpose to effectively use purge gas by using as fuel gas as cell purge gas mainly comprising hydrogen, nitrogen and argon produced in the ammonia synthesis industry. Hereby a purge gas containing hydrogen, nitrogen and argon, and practically free from CO and CO.sub.2 generated in the ammonia synthesis process is supplied to an alkaline fuel cell as a hydrogen source. Use for fuel cell of purge gas is very effective compared with use for boiler. Since the content of CO and CO.sub.2 in purge gas is negligible compared with that in hydrogen from a hydrogen bomb, when purge gas is used as hydrogen source of alkaline fuel cell, generation of Na.sub.2 CO.sub.3 is very small and life of the cell is increased.
Thus, this known method utilizes alkaline fuel cells, whereas the method according to the present invention utilizes Molten Carbonate Fuel Cells or Internal Reforming Molten Carbonate Fuel Cells.